1. Field of the Invention
The present invention relates to a digital television broadcast receiver for receiving so-called digital CS (communications satellite) television broadcasts as well as CATV (community antenna television) transmissions.
2. Description of the Related Art
Conventional digital broadcast receivers utilize in part an analog tuner 11, shown in FIG. 2, for conventional analog television broadcast reception. The analog tuner 11 comprises an input tuning circuit 12, a mixing circuit 13, a local oscillator 14 and an output filter 15. The input tuning circuit 12 is constituted by a single tuning circuit capable of tuning in to the frequency of a received signal representing each channel desired to be received. The tuning frequency is variable so that channels of low through high frequencies may be selected as desired (between about 50 MHz and 900 MHz according to US specifications). The received signal selected by the input tuning circuit 12 is input to the mixing circuit 13 with virtually no attenuation.
The received signal input to the mixing circuit 13 is mixed thereby with a locally oscillated signal from the local oscillator 14 so as to be converted in frequency to an intermediate frequency signal. For conventional analog television systems, intermediate frequency signals are standardized in terms of frequency. Illustratively, in the United States, the video intermediate frequency is fixed at 45.75 MHz and the audio intermediate frequency at 41.25 MHz. As a result, the frequencies of locally oscillated signals from the local oscillator 14 are also determined uniquely; the frequency of any locally oscillated signal is always made higher than that of any received signal by the frequency of the intermediate frequency signal involved. Where the frequency band of received signals ranges from 50 MHz to 900 MHz, the frequency of the locally oscillated signal varies within a range of about 95 MHz through 945 MHz, a 10-fold rate of change.
The intermediate frequency signal from the mixing circuit 13 is forwarded through the output filter 15 and output from the analog tuner 11. The output filter 15 is composed of a band-pass filter having a pass band of about 6 MHz and designed to filter out signals of adjacent channels.
The analog tuner 11 is connected to an orthogonal demodulator 16, a type of digital signal demodulator. An oscillator 17 in the orthogonal demodulator 16 generates a carrier signal (about 44 MHz) to demodulate the intermediate frequency signal, whereby base band signals such as the I signal and Q signal are obtained.
Because conventional digital television broadcast receivers use the traditional analog tuner, the frequency of the carrier signal generated by the oscillator 17 upon demodulation is relatively low (typically tens of MHz) so that harmonics from the oscillator 17 can enter the frequency band of the received signal. For example, the fourth harmonic of the carrier signal is about 176 MHz, which happens to fall within the frequency band of the television signal on channel 7 in the United States. The harmonic, entering the analog tuner 11 and again output from the input tuning circuit 12, interferes with the normal television signal on channel 7.